This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. Goldberg an co-workers have recently prepared a novel low-spin non-heme peroxo complex(Krishnamurthy et al., in press). Vibrational spectra for the complex confirm the presence of an ironbound peroxide with unusually weak Fe-O and O-O bonsd, and UV-visible spectra suggest that the Fe oxidation state is best described as Fe(III). These measurements are unable to define the Fe ligation. In order to define the structure of the complex, and thus establish the understand its role as a mimic for bioinorganic sites, we need to determine whether a pendant thiolate ligand remains coordinated to the Fe. We will use EXAFS to define this structure.